1. Field of the Invention
The present invention relates to interface circuits for supplying a voltage from a control circuit referenced to a first voltage to a terminal not referenced to this voltage. The present invention will more specifically be described in the case where said terminal is a control terminal, but is not limited to this application.
2. Discussion of the Related Art
Interface circuits or voltage shifting circuits are used especially when a switch comprising two switches in parallel, head-to-tail, connected to a circuit supplied by an A.C. voltage is desired to be controlled. Generally, the control terminal of one of the two switches is referenced to a reference voltage and the control terminal of the other switch is referenced to a terminal which follows the variations of the A.C. voltage. To solve this problem, isolation circuits, for example, transformer or optocoupler circuits, are often used. Various interface circuits ensuring an isolation between the control terminal likely to be at a variable voltage and the control circuit are also known. However, a frequent disadvantage of such circuits is that they are not integrable, especially when the voltage on the control terminal which is desired to be driven can be high, for example, on the order of the voltages available on commercial supply networks (for example, 220 volts RMS).
FIG. 1 shows an example of a known interface circuit. A power circuit comprises an A.C. voltage source VAC supplying a load L in series with a bidirectional switch formed of two parallel voltage-controlled switches monodirectional for the current, MBS1 and MBS2, each of these switches being able to stand a high forward or reverse voltage. These switches are, for example, insulated-gate bipolar transistors (IGBT), manufactured in a well technology. It is assumed that the terminal of switch MBS1-MBS2 opposite to the load is connected to a reference voltage G. In the considered example, each of transistors MBS1 and MBS2 is controllable by a positive voltage applied between its gate and its emitter. Switch MBS2 has its emitter connected to reference terminal G. Thus, a control signal provided by a control circuit referenced to terminal G may be applied on its gate g2 with no specific precaution. However, switch MBS1 has its emitter connected on the side of variable A.C. voltage VAC. The control order must be applied when the A.C. voltage is negative with respect to terminal G but, due to the presence of a gate-emitter resistance RGE and of a protection diode Z, gate g1 of transistor MBS1 is periodically at a high positive voltage from which the control circuit must be protected.
An example of a control circuit comprises a control block 10 and an interface circuit 20. Control block 10, for example, comprises a resistor R1 in series with an N-channel MOS transistor NMOS, between a supply voltage Vcc, for example, on the order of 10 volts, and reference ground G. The gate of transistor NMOS is connected to a terminal 11 which receives a control order, for example, the output of a microprocessor, and turns it, at the connection point between resistor R1 and transistor NMOS, into a signal CTRL likely to control interface circuit 20. The interface circuit comprises, between a terminal at voltage Vcc and gate g1 of insulated-gate transistor MBS1, the series connection of a P-channel MOS transistor PMOS, of a resistor R2, and of a diode D1. Diode D1 is necessary to avoid that, when terminal g1 is at a high positive voltage, this positive voltage is sent to the control circuit connected to terminal 11 and to supply voltage source Vcc, which could be destructive for these elements. Indeed, transistors PMOS and NMOS in conventional configurations are not capable of blocking a reverse voltage.
It should be noted that the circuit of FIG. 1 only is an example of a control and interface circuit of prior art and that many other circuits may be provided to ensure similar functions. Bipolar transistor circuits could, for example, be envisaged instead of MOS transistor circuits. A feature common to this type of circuits is that they comprise in series a MOS or bipolar transistor likely to stand a forward voltage and a diode D1 likely to stand a reverse voltage of several hundreds of volts. In practice, there is no known manner of monolithically integrating such circuits, which are generally formed as discrete components, especially as concerns diode D1.